HISTORY
OF OCEAN WAVE RECORDING
IN
SOUTH AFRICA

Wave data have been collected around the
Southern African coasts since about 1940. The first wave
observations were made from merchant vessels which travelled
around the coast; the so-called Voluntary Observing Ships
(VOS). From these ships visual estimates were made regularly
of wave heights, periods and directions. These observations
were later augmented by wave recordings made with a variety
of instruments including clinometers, ship-borne wave records,
pressure meters, inverted echo-sounders and accelerometer buoys.

Wave data for use in coastal engineering
projects in South Africa were first collected by means of a
wave clinometer off the Bluff at Durban in April 1961. The
wave clinometer consists of a telescope having graduations
on one lense through which nearshore wave direction is
observed from a high vantage point on the shore. Visual
estimates of wave height and wave period can also be made
by observing the movements of a moored buoy through the
graduated lense of the telescope.

During the period 1964 to 1969 the
research ships Africana II, Thomas B Davie, Meiring Naudé
and Benguela, as well as the survey vessel SAS Natal, were
fitted with NIO ship-borne wave recorders. These recorders,
in which use is made of a combination of accelerometers
and pressure recorders, were fitted to the ship's hull and
gave as output an analogue trace of the water surface from
which wave-height and wave-period data could be extracted.
Wave directions were estimated visually from the bridge of
the ship by means of the ship's compass.

South Africa also operated a weather ship,
the F H Hughes during the period September 1969 to March
1974. The ship was usually stationed off Cape Point at
40° S; 10° E. It was first fitted with a Boersma recorder
which was later replaced by a NIO recorder.

In February 1967 a wave research group
was established within the National Mechanical Engineering
Research Institute (NMERI) of the CSIR with the main aim
of recording and statistically analysing wave conditions
along the coastlines of South Africa and South West Africa
(Namibia). This so-called "Ocean Wave Research Project" was
initiated by the South African National Committee for
Oceanographic Research (SANCOR) with financial support
and guidance from SANCOR, VISKOR, SOEKOR and NMERI.

The main wave recording instrument used
during the period 1961 to 1970 was the wave clinometer. The
number of wave clinometer stations was increased from one
in 1965 to twelve in 1970. The wave clinometer stations
remained in service until late in 1974 when measurement by
this means were largely stopped.

In an attempt to increase the accuracy
of the visually measured wave data obtained by the
clinometer, a number of wave recording instruments were
experimented with during the period 1966 to 1969. First an
inverted echo-sounder (Kelvin Hughes), connected by cable
to shore, was tried in Cape Town and Durban. This experiment
was eventually abondoned mainly because of difficulties with
the laying and maintenance of the cable through the
surf-zone. A self-contained inverted echo-sounder (INES) was
developed and used for a while, but it suffered from many
internal defects and leakage problems. Eventually it fell
into disuse without producing many useful results.

A few pressure recorders (OSPOS) were
bought from Van Essen in Holland by NMERI and VISKOR and
this self-contained unit proved to be very reliable. The
records were, however, difficult to analyse and doubts were
expressed regarding the transfer functions used to convert
the pressure-record to a water-surface record.

In July 1969 an accelerometer buoy, the
Datawell Waverider was installed in 100 m water depth
off Mossel Bay. The moored buoy transmitted to shore where
the data were recorded. Initially a paper tape recorder was
used, but it never functioned satisfactorily and later
recordings were made on a strip-chart recorder. The
Waverider soon proved to be superior to any of the wave
recorders tried previously and, during the period 1971 to
1973, the number of Waverider stations was increased from
one to seven. All of these stations recorded waves in
analogue form on paper rolls, usually for 20 minutes every
six hours. All analysis had to be done by hand and by late
1974 the backlog in analysis became so large that it was
decided to reduce the number of Waverider stations to those
required for urgent coastal engineering studies until
instrumentation and software could be developed for
recording in computer-campatible form. The first digital
recorder was installed at Slangkop, off the Cape Peninsula,
in February 1976. Digital recordings were made on a cassette
tape, typically with a capacity of ten days worth of four
recordings per day.

Although the digital recordings could now
be quickly processed by computer, the need was recognised to
develop software which would check the quality of the data
without having to display or plot the individual recordings
and rely on human judgement as to its integrity. A joint
project was launched by the Institute of Maritime Technology
(IMT) and the National Research Institute for Oceanology
(NRIO, CSIR) and in 1980 a computer program was produced
which efficiently analyses and checks wave recordings for
quality. A computerized database was also established for
storage and retieval of all the wave records which had passed
the quality checks.

Recording on cassette proved to have several
drawbacks. Recording stations had to be situated at manned
locations to facilitate changing and mailing the cassettes to
the CSIR in Stellenbosch. Cassettes were sometimes lost or
damaged and often accumulated at the recording site. The
result was that often bad recordings and lost Waveriders were
only discovered many weeks later with resultant loss in data
and equipment. With the advent of the PC-era, a solid-state
datalogger was developed and connected, via dial-up modem,
to a PC-based base station at Stellenbosch. Software was
developed which automatically collected and analysed the
data on an around-the-clock basis. This system became fully
operational around 1985.

Work also continued to locally produce
an accelerometer buoy. The final product
(Wavemonitor)
was developed and successfully implemented around 1993.

Although the accelerometer buoy has
virtually become the standard wave recording instrument
in South Africa, it does not measure wave direction. During
the early 70's VISKOR developed the DOSO. This instrument
sensed the direction of the orbital motions of the waves and
is normally placed on the sea-bottom in a water depth of
less than 20 m. Useful shallow water data were obtained from
the DOSO at a few sites in South Africa such as Koeberg and
Gansbaai. Radar was also used to record nearshore wave
direction at Koeberg and useful (although intermittent)
information was obtained by analysis of photographs of the
image on the radar screen. Since the DOSO and radar were
used to measure waves in relatively shallow water, refraction
techniques were required to obtain the more generally
applicable deep-sea wave direction.

Field experiments were also conducted
with commercially available directional wave buoys.
Between 1986 and 1992 field tests were done with an Endeco
Type 956 Wavetrack buoy and a Datawell directional Waverider
buoy. The data from these two buoys were compared to those
of an electromagnetic currentmeter (EMCM), placed underneath
the buoys. Results indicated that the directional Waverider
and the EMCM compared well. It was established that the
Endeco buoy had difficulty in resolving the directional wave
spectrum in a swell dominated wave climate such as around
the South African coast.

Although non-directional Waverider and
Wavemonitor buoys were mainly used, there was a growing
need for real-time directional wave information. At the
time, the CSIR successfully developed a directional wave
buoy using standard GPS technology, called the
3D buoy.

Directional measurements commenced
in 1997 off East London and later off Richards Bay and
Cape Town. However, due to poor continuous satellite
coverage at the time, the buoys were replaced with
Datawell directional Waveriders. Bottom-mounted systems
were also acquired. A
Teledyne RD
Instruments ADCP has been operating on a real-time
basis since 2002 off the Port of Durban. This system
provided information on the ocean currents as well as
the wave conditions.

As the need grew for the real-time data,
the basic wave recording had to be automated in order to
provide an efficient and cost-effective system to all the
major South African ports. The development of this system
led to the wave recording network (Wavenet) whereby the data
from the wave stations around the South African coast could
be effectively managed.

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